The tragic mass killings on Monday at the Washington, D.C., Navy Yard appear to be unrelated to terrorism and will probably turn out to be another appalling but isolated act by a troubled individual whose private motives may never be known.

It follows a number of highly publicized mass killings in recent years, but whether it or they represent an identifiable and special failure of public policy is an open question. And as the Pew Research Center notes, such shootings rivet national attention but represent a very small share of firearms violence.

A recent study by the Congressional Research Service found that there were 78 incidents of public mass shootings between 1983 and 2012, killing 547 people and injuring another 476. A “public mass shooting” refers to those that happen in public and have at least four victims chosen somewhat indiscriminately. To put these numbers in perspective, in Chicago there were 500 murders in 2012 alone, according to FBI statistics.

In the horrific incident in Washington, 12 people working at the Naval Sea Systems Command headquarters were killed by the gunman, Aaron Alexis, who was then killed by police after a gun battle lasting more than half an hour. Another eight persons were injured, three by gunfire.

Among the questions still not fully answered are the state of the gunman’s mental health, his motive and how he was able to bring a shotgun into a secure Navy facility.

The apparent answer to the last question is that Mr. Alexis, a Navy veteran, carried a pass allowing him to drive into the Navy Yard in his capacity as an employee of a contractor working for the Navy. The Navy has announced that it is reviewing its base security procedures.

His employer has said Mr. Alexis would not have been hired and given a security clearance if his troubled background and brushes with the law had been a part of his Navy record.

According to The New York Times, investigating officials have found that Mr. Alexis had showed a pattern of mental instability for more than a decade.

On one occasion he was arrested in Fort Worth, Texas, for shooting through the ceiling of his apartment into the apartment of an upstairs neighbor with whom he had an altercation. Even after that incident, the Navy gave him an honorable discharge.

This is a sad but familiar tale: A troubled individual somehow eludes badly needed psychiatric treatment even after exhibiting dangerous behavior, with no red flags having been attached to his record warning of potential trouble to come.

With a population of more than 310 million, in a nation that values individual liberty, there are undoubtedly more than a few such troubled and dangerous individuals.

As Americans keep finding to our sorrow, these rare cases can lead to major tragedy.